CMOS image devices are widely used as solid-state image sensors. A unit pixel of a CMOS image device includes MOS transistors that can be embodied in a relatively small area to thereby provide high resolution. Moreover, processing logic can be integrated with the unit pixels, which means the image circuit and the signal processing circuit can be integrated on one chip.
A unit pixel of the CMOS image device includes a photodiode for sensing images. In addition to the photodiode, each unit pixel includes a transfer transistor, a reset transistor and an amplifying transistor, to transfer the signal sensed by the photodiode to a signal processing circuit. Unfortunately, the CMOS image sensor with the foregoing structure may support parasitic dark currents due to structural problems. The dark current may be caused by dangling bonds occurring on an interfacial surface between layers of different materials (e.g., an interfacial surface of a silicon substrate and a silicon oxide layer). The dangling bonds extend between a valence band and a conduction band in an energy band, and may trap electrons regardless of the supply of light causing electron generation in the conduction band. The trapped electrons may be accumulated in the photodiode and cause the generation of a white spot on a picture as an image is acquired. This white spot may take the form of a ghost image.
In order to reduce an influence of the dangling bond that incites the dark current as described above, a p-type impurity region may be formed in the interfacial surface between layers of different materials (i.e., at the surface of the substrate, where the CMOS image sensor is formed). Such a CMOS image device is disclosed in U.S. Pat. No. 6,730,899.
FIG. 1 partially illustrates a CMOS image device having a p-type image region. Referring to FIG. 1, the CMOS image device 10 includes a photodiode 20 for converting light to charge, formed within a semiconductor substrate 15, and a transfer transistor 30 transferring the charge sensed by the photodiode 20 to a floating diffusion region 40. Also, a reset transistor 50 resets the floating diffusion region 40, and an amplifier 60 buffers a voltage of the floating diffusion region 40. The photodiode 20 includes an n-type photodiode region 20a and a p-type photodiode region 20b disposed thereon. The floating diffusion region 40 is composed of a high-density n-type impurity region. The transfer transistor 30 includes a transfer gate 35 disposed between the photodiode 20 and the floating diffusion region 40. In this case, the transfer gate 35 is insulated from the semiconductor substrate 15 by a gate oxide layer 32. The transfer gate 35 transfers the charge generated from the photodiode 20 to the floating diffusion region 40. A p-type impurity region 70 is formed opposite the transfer gate 35 (i.e., on the entire surface of the substrate between the photodiode 20 and the floating diffusion region 40), to get rid of an influence of the dark source (i.e., dangling bonds). The impurities in the p-type impurity region 70 inhibit the dark source current.
In the conventional CMOS image device, the n-type photodiode region 20a of the photodiode 20 may extend underneath the transfer gate 35 to inhibit an image lag. However, the extension of the n-type photodiode region 20a may induce a potential barrier (refer to the lower portion of FIG. 1) underneath the transfer gate 35, which extends toward the floating diffusion region 40. The slope of the potential barrier towards the photodiode 20 may become abrupt. Due to this fact, electrons which may remain on the interfacial surface of the substrate 15 and the gate oxide layer 32 may be forced back towards the photodiode 20, thereby causing the dark current.